On cooling
http://bobhooversblog.blogspot.com/search?q=coolingGone now, but "Veeduber" flew an Aeronca C-3 with an 1100 cc VW BEFORE 1960, and may have been the first person in the USA to fly behind a VW. He built and sold VW aeroengines for a while, had strong opinions, and was a friend and a mentor to me. Also check out his HVX modifications, to put more oil in the heads around the valves to carry off heat, for the VW is as much Oil Cooled as Air Cooled. A typical Aerovee DOES NOT have these mods, mine has a FEW of them.
Pressure differential between the upper and lower plenum should be about 9 inches of water at full power, which a well designed and implemented "firewall" forward should be able to easily achieve at about 100 MPH.
John Monnett has a lot of hours behind VW aeroengines, and had the help of Claude of Claude's Buggies (now CB Performance) and his sons, both pilots, in the development of the ORIGINAL AeroVee (for the Sonerai) and the AeroVee 2.0, but he did some things that made me wonder. The first iteration of baffles on the Sonex Aerovee were Not Very Good; the current "fence" baffles are much better, IMHO. The needed lip for the cowling outlet is NOT in the plans set, but is rather in the instructions for the cowling, and are easy to miss. The oil cooler inlet for the Cooler Under The Sump arrangement has a tendency to pressurize the LOWER plenum, sapping the pressure differential. Someday, I'll change my airplane over to a top mount cooler, but I want to fly it first - we are close. This year.
Another issue has been the heads themselves, particular the aftermarket, drag racing oriented ones. They often did not have adequate airflow around the exhaust valves. The heads AeroConversions sells now are based on Chinese sourced EMPI castings and are as good as the Original German heads that have been unavailable for 25 years.
Searching this forum and a lot of Aerovees have burnt valves and cracked heads in frightfully short periods of time, but plenty of others have gone hundreds of hours with little trouble.